Method and device for wire patenting by radiation-convection heat transfer

ABSTRACT

A method and device for cooling a wire. The device includes a block of material having a very high thermal capacity with a channel adapted for allowing passage of a wire to be cooled and at least one conduit for circulation of a cooling fluid, and at least one nozzle for injecting a turbulent fluid jet towards the wire to be cooled. The turbulent jet is capable of producing necessary cooling, thus avoiding the need for using conventional lead baths.

FIELD OF THE INVENTION

The present invention is applied to wire patenting. It more specifically relates to a method and a device for high-carbon wire patenting.

BACKGROUND OF THE INVENTION

In wire production processes, the starting steel (in the form of a wire rod) is drawn. The drawing operation gives the material metallographic and mechanical properties that are not advisable for subsequent use thereof. For this reason a patenting step is necessary, which again gives the wire the suitable characteristics for either continuing the process or as an end product.

Patenting is an isothermal transformation heat treatment consisting of austenitization of the steel around 900° C. (it can vary depending on the carbon content) and rapid cooling to 550° C. The result is a fine perlite structure (troostite) giving the wire high strength as well as good ductility. Currently, most wire manufacturers use high-temperature fluidized bed or open flame furnaces and lead baths in the rapid cooling step for patenting.

The use of lead in cooling means that it appears as a contaminant in subsequent steps (cooling the wire in water, oxide cleaning with acids, washing, even in the zinc bath in the case of being galvanized). This classifies the waste as special, making it necessary for a waste management company to treat and eliminate it. The high toxicity of lead thus makes it necessary to search for alternatives.

Therefore, in the search for new patenting processes it must be taken into account that they should be environmentally sustainable and energy efficient, as well as not harmful for users.

Patent ES 2039708 T3 describes a wire patenting process using one or several tubes filled with a gas, devoid of forced ventilation, modulating the heat exchanges throughout the cooling path of the wire and varying the dimensions of the tubes, their length and in-line arrangement. The process described in this document is a process for heat transfer based on natural convection in a gas and the subsequent heat conduction through the wall of the tube to the cooling fluid circulating through a coaxial annular channel. This process presents the problems of having low energy efficiency, deficient heat modulation, complex adaptability to wires of different diameters, the considerable length of the device for reaching the desired degree of cooling of the wire, and the high cost of the installation. In particular, as can be inferred from reading the description of the system, the heat transfer during the cooling phase depends almost exclusively on the flow rate of the cooling fluid and its log mean temperature. A minor log mean temperature difference must result from the discussed process for heat transfer; accordingly, in order for the specific flow of heat through the wall of the tube in internal contact with the gas to be large, the necessary flow rate of the cooling fluid must be high; and it must be borne in mind that water is a scarce resource. On the other hand, since the inert gas which fills each sector of tube is virtually immobile, it will be progressively heated, accumulating heat, which is in detriment to the efficacy of the process for the heat transfer from the wire to the cooling fluid.

OBJECT OF THE INVENTION

These drawbacks and problems among others are solved by the system and method for cooling wires of the invention. The invention proposes a method for wire patenting comprising a cooling step, and where said cooling step occurs by means of applying a turbulent fluid jet towards the surface of the wire. The turbulent jet is preferably produced by at least one flat jet nozzle situated such that the jet is perpendicular to the surface of the wire.

The method optionally comprises an in-line heating step for heating the wire, before said cooling step, which is used to reach the austenitization temperature of the wires circulating therein. It can further comprise a drawing step before entering the system for heating and a prior cleaning step, whereby all the residues of lubricants from the previous drawing step are eliminated. A system for heating by means of electromagnetic induction currents individually wire-by-wire can be used in the heating step. The entire transit of the wire in the process is preferably done in complete absence of oxygen.

The invention also relates to a device for carrying out the methods described above. Said device comprises a block of material having a very high thermal capacity with a channel adapted for allowing the passage of a wire to be cooled and at least one conduit for the circulation of a cooling fluid, and it further comprises at least one nozzle capable of injecting a turbulent fluid jet towards the wire to be cooled. The nozzles are preferably flat jet nozzles and the device is axially symmetrical. It optionally comprises means for modulating the intensity of the heat transfer from the wire. The number of nozzles is also preferably predetermined depending on an assigned rate of cooling and they are oriented according to radii perpendicular to the main axis of the block.

As a result of the device and method of the invention, the current processes for wire patenting, which use sulfuric/hydrochloric acid in their cleaning systems and lead in their cooling baths, and consume a large amount of energy, are replaced.

The number of nozzles, their geometric dimensions, length, width of the outlet groove, cone angle, etc., as well as the relationship between them, and their orientation with respect to the normal to the surface of the wire can vary according to if there are needs of the process for convection heat transfer from the hot wire.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of aiding to better understand the features of the invention according to a preferred practical embodiment thereof, the following description is accompanied by a set of drawings in which the following has been depicted by way of illustration:

FIG. 1 is a general scheme of the system for cooling patented wire object of the patent application.

FIG. 2 shows a cross-section view and a longitudinal view of one of the possible configurations of nozzles, gas conduits and cooling fluid conduits which respond to the fluid dynamic and heat transfer requirements described above.

FIG. 3 shows an alternative example of the invention, but it maintains the same operating principle.

FIG. 4 is a graph showing how the non-uniformity of the flow over the object translates into a non-uniform distribution of temperature and of the heat transfer over its surface.

DETAILED DESCRIPTION OF THE INVENTION

The patenting process comprises preferably a drawing step for drawing the wire, a cleaning step for removing possible residues of lubricant used in the previous step and an in-line heating step for heating the wire to the austenitization temperature. After heating, cooling occurs without the need for lead baths.

As a complement to the arguments set forth above, the information about the physical phenomena on which the system for cooling the wire by heat transfer from it by the combined processes of radiation-convection and conduction can be extended.

To extract heat from the wire without contact with a cold solid surface, from a practical industrial point of view only the processes of radiation and convection heat transfer can be considered.

Given a wire surface at a high temperature, the intensity of the heat emission by radiation depends on its temperature and on the temperature of the receiving surface in relation to the wire, both to the fourth power, on the composite emissivity and on the view factor, besides the value of the Stefan-Boltzmann constant. Accordingly, in the case at hand, the major variable is the temperature of the receiving surface.

Assuming a heat capacity of the material of the solid surface, its temperature will depend on the efficiency with which it is cooled. Said cooling can be achieved simply by heat conduction through the solid material towards the surfaces in contact with the cooling fluid, by the combination of said conduction with a process for the forced convection produced by blowing said surface with a gas that is at a lower temperature.

It is obvious that the cooling capacity of the process for heat transfer by the association of heat conduction and forced convection is considerably greater than the cooling capacity by heat conduction alone.

The capacity of forced convection heat transfer is characterized by its Nusselt number. Of all the techniques for applying forced convection for heat transfer in industrial processes, the one that has been proven most efficient is the use of fluid jets, whether the fluid is a gas, a liquid or a gas-liquid mist, having a high turbulence intensity, which is achieved by means of nozzles, mainly those referred to as flat jet nozzles. The flat jet nozzle, the longitudinal groove of which coincides with the direction of the axis of the cylindrical body on which the gas jet is projected, is the optimal configuration for the following reasons:

-   -   1. The ratio of the distance of the nozzle discharge section to         the surface that receives the jet with respect to the width of         its groove is constant throughout the entire area of action.     -   2. The core of the flow, i.e. the width of the jet in which the         speed of the ejected fluid is maximum, is constant throughout         the entire area of action.     -   3. The hydraulic diameter of the nozzle discharge section         involved in the definition of the Reynolds number is small         compared to that which corresponds to other geometric         configurations with an identical area of the outlet opening.

As a result of the previous characteristic, the regimen of the current in the jet is two-dimensional, the turbulence intensity is very high and its distribution is spatially uniform. This results in a high capacity for heat and momentum transfer in the surface which the jet strikes.

The two-dimensional characteristic of the outlet groove of the nozzle and its longitudinal orientation facilitate the evacuation of the jet once it strikes the surface of the solid with which it exchanges heat, directing it towards the surfaces of the enveloping wall, cooling them.

High Nusselt number values are thereby achieved, these numbers being given in the case of flat nozzles by

${Nu}_{m} = {c \cdot {Re}^{m} \cdot \Pr^{n} \cdot \left( \frac{\Pr}{\Pr_{0}} \right)^{p}}$

wherein c is a numeric constant dependent on the geometric configuration of the nozzle-contour surface, Re is the Reynolds number, Pr is the Prandtl number and m, n and p are numeric coefficients which depend on the shape and dimensions of the nozzle, as well as the orientation of the jet with respect to the normal to the surface which the fluid strikes, and very dependent on the ratio between the distance from the nozzle discharge section to the surface receiving the jet and the hydraulic diameter of the latter.

In the system for cooling wire object of the invention, it is precisely this process of forced convection heat transfer by means of flat nozzles having a highly turbulent flow that contributes to a large extent to the intensification of the heat transfer, because not only does it activate the direct cooling of the wire but also of the entire surface receiving the radiant flux emitted by the wire and reduces part of the heat conducted by the solid mass towards the cooling fluid, whereby the length of conduit necessary for cooling the wire and the required consumption of cooling fluid—water—is considerably reduced. The claimed system for cooling has the novelty of using a highly efficient forced convection circuit which incorporates flat nozzles generating very turbulent gas jets, the flow rate and temperature of which can be regulated at will. These jets and the gas reflux resulting after striking the surface of the wire assure that not only is a very high Nusselt number obtained in the heat exchange with the wire, but also the control of the temperature of the enveloping tube which in turn controls the radiant flux of heat from the wire and, in summary, the reduction of the flow rate of the cooling fluid necessary, as well as the reduction of the length of the installation.

The device for heat treatment of the invention is a device for heat transfer by the suitable combination of radiation, convection and conduction, preferably being axially symmetrical, for example cylindrical. It is made up of a continuous channel or a channel formed by several consecutive sectors having different dimensions aligned according to one and the same axis, provided with several radially oriented flat nozzles through which a gas, or a mixture of gases, a finely sprayed liquid or a mist is ejected in a highly turbulent regimen at a temperature that can be externally regulated.

The device is formed by a block of material (FIG. 1) the thermal capacity of which is very high, in which there are several conduits 5 for feeding fluid to the nozzles 1, whether for the subsequent extraction from the chamber or for the circulation of cooling fluid for the purpose of controlling the temperature of the material of the block and, accordingly, for regulating the radiation-convection heat transfer of the solid which is displaced at an speed which can be regulated through the inside of the block through a channel 9 (FIG. 2).

It furthermore has means for modulating the intensity of the heat transfer from the solid in motion through the external control of the temperature of the gas, of the cooling fluid, and of their respective flow rates. According to the elements detailed in FIG. 1, the operation is as follows:

The flat nozzle 1 described in FIG. 1 is used to eject a turbulent gas jet towards the wire traversing the tube. Once the gas has struck the surface of the wire, it is oriented towards a chamber 2, which is used to recirculate said gas. In the system, the gas is introduced in the chamber by means of the impulsion of a gas blower 3 that has variable-speed and is under regulated pressure and flow rate. Said gas is introduced at a temperature controlled by means of the system 4 for controlling the programmed temperature of the gas. The system is cooled by means of the cooling conduits 5 (of the recirculated gas, of the tube for receiving the radiation emitted by the wire and the solid structural parts of the system). The cooling fluid impelled by a circulating pump 6 for circulating the fluid of the cooling circuit, with variable speed regulation for controlling the flow rate, circulates through said cooling conduits. Said system for cooling includes a programmed regulation of the temperature 7 of the cooling fluid.

The modulation of the intensity of heat transfer, given a rate of passage of the wire through the device for cooling, is achieved by regulating the temperature of the gas ejected by the flat nozzles over the wire by means of the system 4, regulating the mass flow of gas or varying the operating speed of the gas compressor, or acting on both.

In addition to the previous basic modulation action, it is also possible to vary the flow rate and temperature of the cooling liquid, the equipment for controlling the temperature 7 of the cooling liquid, and the flow rate of the liquid impelled by the pump 6.

The system is designed such that means such as mixing chambers, mist chambers, sprayers, etc., can be incorporated so that the fluid of the jets projected through the flat nozzles is a mixture of gases, a mist, a sprayed liquid or a chemical vapor serving either for the heat transfer effects or for reactive-chemical effects on the surface of the solid in motion, for example: descaling of metal surfaces by acid, Cr—Ni passivation of steel surfaces by means of nitric acid mist, bonding reactions in the interface of composites, etc.

The number of nozzles necessary is a function of the rate of cooling of the wire assigned to the convection process. Once this rate has been fixed, the Nusselt number value is determined and, from the latter, the Reynolds number is calculated. The Reynolds number is expressed as Re=Vd_(h)/ν where d_(h) is the hydraulic diameter of the nozzle discharge section, V is the velocity of the fluid therein, and ν is the kinematic viscosity of the fluid.

The Reynolds number is a dimensionless parameter of the relative measurement of inertia forces with respect to the viscous forces in a fluid current. The value of the Nusselt number, which in turn defines the heat transfer coefficient, depends on the value of this parameter.

Once the Reynolds number is known, a fluid dynamic optimization process is developed in which the nozzle length, the width of the nozzle discharge section and the separation between them are interactively involved, the number thus being determined. The optimization process includes comparing the analytical results obtained by applying the available empirical correlations.

The orientation of the nozzles in the most important applications is defined by the direction of the jet ejected, usually according to the line that is normal to the surface it strikes, in the case at hand, the surface of the wire. Nevertheless, other orientations can be applied in the search for a greater surface of contact of the jet with the surface of the wire, there being a compromise between said orientation and the uniformity of the field of temperatures in the surface being struck.

FIG. 4 shows how the non-uniformity of the flow over the object translates into a non-uniform distribution of temperature and of heat transfer over its surface.

The mass flow of gas and its temperature are externally regulated according to the scheme of the system shown in FIG. 1. The mass flow is regulated by varying the speed of the drive motor of the blower according to a routine which is determined by the characteristic curve of the blower installed. The signal necessary for applying the regulation routine comes from one or two pressure sensors installed in the gas circuit. The temperature of the gas is regulated by means of an external heat exchange the flow of cooling fluid of which is established by means of a routine the signal of which is from the thermocouples installed in the gas circuit. The regulation can be on-off, proportional or proportional-integral, according to the desired precision for the value of the temperature of the gas at the nozzle discharge.

On-off control is understood as all-nothing, e.g., a reference temperature is fixed in the N₂ circuit, when the thermocouple for measuring the temperature at the outlet of the blower detects a temperature difference with respect to the reference temperature, a signal is produced whereby acting on the external heat exchanger by completely closing or opening the valve for the passage of water through the exchanger (a step regulation).

The differential regulation is implemented using the temperature difference read in the N₂ current, before the heat exchanger and after the blower and, according to the proportional band of the regulator, the valve for the passage of water through the exchanger is opened or closed proportionally.

In integral control, the measurement of the temperature difference and that of the flow rate impelled by the blower are combined to integrate them by means of a routine which determines either the regulation of the flow rate of the blower, the temperature difference upon passing through the external exchanger, or both in order to reach a maximum energy efficient operating state. 

1. Method for wire processing comprising a cooling step that occurs by applying a turbulent fluid jet towards a surface of the wire.
 2. Method according to claim 1, wherein the turbulent jet is produced by at least one flat jet nozzle situated such that the jet is perpendicular to the surface of the wire.
 3. Method according to any of claim 1, further comprising an in-line heating step for heating the wire to the austenitization temperature before the cooling step.
 4. Method according to claim 3, further comprising a drawing step before heating.
 5. Method according to claim 4, further comprising a prior cleaning step, whereby residues of lubricants from the previous drawing step are eliminated.
 6. Method according to any of claim 3, wherein the heating occurs by electromagnetic induction currents individually wire-by-wire.
 7. Method according to any of the preceding claims, wherein the processing of the wire is complete absence of oxygen.
 8. Device for cooling a wire, comprising: a block of material having a very high thermal capacity with a channel adapted for allowing passage of a wire to be cooled and at least one conduit for circulation of a cooling fluid; and at least one nozzle for injecting a turbulent fluid jet towards a surface of the wire.
 9. Device according to claim 8, wherein the at least one nozzle is a flat jet nozzles situated such that the injected jet is perpendicular to the surface of the wire.
 10. Device according to claim 8, wherein the device is axially symmetrical.
 11. Device according to claim 8, further comprising a device for modulating the intensity of heat transfer from the wire with respect to a rate of passage thereof.
 12. Device according to claim 8, wherein a number the at least one nozzle is predetermined depending on an assigned rate of cooling.
 13. Device according to claim 8, wherein the at least one nozzle is a plurality of nozzles oriented according to radii perpendicular to a main axis of the block. 